1. Field of the Invention
The present invention relates to a method of acquiring a diagnostic index, and in particular to a method of acquiring a diagnostic index that indicates a condition of obesity of a subject based on slice images of the abdominal area of the subject.
2. Description of the Related Art
Various techniques for measuring a body fat ratio indicating a ratio by weight of the body fat to the body weight have been known. Examples of such techniques to obtain the body fat ratio include: estimating the body fat ratio based on a body density that is obtained by measuring a difference between body weights of a subject human body under water and on land; and an impedance method in which the body fat ratio is estimated utilizing the fact that an electric resistance of a human body when a very small electric current flows therethrough varies depending on the amount of the body fat. It should be noted that the body fat is a collective term for fats forming a human body and includes the visceral fat and the subcutaneous fat.
Further, a technique for estimating a ratio of the visceral fat, which is a part of the body fat, to the human body is known (see Japanese Unexamined Patent Publication No. 2004-254933). In this technique, a slice image of a human body, which allows clear identification of a boundary of a visceral fat region, is obtained through tomographic imaging of the human body using a X-ray CT apparatus, and the ratio of the visceral fat to the human body is calculated from an area of the visceral fat region and an area of other regions in the slice image. In this technique, the visceral fat ratio is estimated from a single slice image that is taken along a plane crossing the umbilicus of the human body.
Furthermore, a technique for acquiring diagnosis information that indicates a condition of obesity of a subject is known. The diagnosis information is acquired based on areas of a visceral fat region and a subcutaneous fat region of the subject that are found from a singe slice image representing a cross-section of the abdominal area of the subject (see U.S. Pat. No. 6,766,272).
Since it is difficult to directly measure the body fat ratio, the visceral fat ratio and the subcutaneous fat ratio, these values are estimated indirectly from various measurement data, as described above.
There are differences among individuals in positional relationships between a cross-section of the abdominal area and the visceral organs of a subject, or between the cross-section and sites where the body fat tends to accumulate. Therefore, in a case where the diagnostic index is acquired from a slice image that represents a single cross-section taken along a plane crossing the umbilicus, a diagnostic index acquired for a certain subject and a diagnostic index acquired for another subject, whose diagnostic index should be similar to the diagnostic index of the first subject, may not be similar to each other. Thus, there is a demand for reduction of such discrepancy in the diagnostic indices due to differences in body structures of individual subjects.